1. Field of the Invention
The present invention relates to an image heating apparatus which is suitable for a heat fixing device mounted in a copying machine or a printer, and to a pressure roller used in the apparatus.
2. Description of the Related Art
On an electrophotographic copying machine or printer, a fixing device is mounted which heats and fixes toner images formed on a recording material. There are various types of heat fixing devices including: a heat roller type which heats and fixes an image while sandwiching and transporting a recording material by a fixing roller heated by a halogen heater provided therein and a pressure roller; an on-demand type (also called a film-heating type) which contacts a ceramic heater with the inner surface of a flexible sleeve (a fixing film or a fixing belt) basically made of a heat resistant resin and a metal and heats a recording material through the flexible sleeve; and an electromagnetic induction heating type in which a rotor itself to be contacting with the recording material generates heat.
When a small size of paper is continuously printed with an image-forming device mounting such a heat fixing device, there occurs a phenomenon of slowly increasing the temperature at the area of a fixing nip portion in a longitudinal direction, through which the paper does not pass (temperature rise in a no-paper-passing area). If the temperature of the no-paper-passing area is too high, it causes damage in each part of the apparatus, and when a large size of paper is printed in a temperature risen state in the no-paper-passing area, it causes a high-temperature offset in the region corresponding to the area through which a small size of paper has not passed.
Particularly, a film-heating type capable of employing a heating body with a low heat capacity has a smaller heat capacity of the heating body than that in a heat roller type, so that it causes a higher temperature rise in a no-paper-passing part of the heating body, and easily causes the degradation of durability, a high-temperature offset and problems such as the instability of film driving and the fold of a film.
In addition, as an image-forming device has a higher processing speed, it causes a temperature rise more often in the no-paper-passing area. That is, as long as a period of time when a recording material passes through a fixing nip portion becomes shorter with an increasing speed, it is inevitable to increase a heat fixing temperature. Also, as long as a period of time when a recording material does not exist in a fixing nip portion (so-called an empty period between sheets of paper) in a continuous printing step is decreased with the increasing speed of an apparatus, it is difficult to uniform the temperature distribution during the interval between sheets of paper.
As one of means for decreasing a temperature rise in a no-paper-passing part, a technique of increasing the thermal conductivity of a pressure roller is generally known. The means aims the lowering of the temperature in the no-paper-passing part through positively improving the thermal conductivity of an elastic layer of the pressure roller, or equivalently, the effect of decreasing the difference of temperature among areas in a longitudinal direction.
For instance, Japanese Patent Application Laid-Open No. H11-116806, H11-158377 and 2003-208052 disclose a method of adding a highly heat conductive filler such as alumina, zinc oxide and silicon carbide to a base rubber for the elastic layer of a fixing roller and a pressure roller, in order to enhance the thermal conductivity of them.
In addition, Japanese Patent Application Laid-Open No. 2002-268423 discloses a method of making an elastic layer of a rotor (which is not a pressure roller but a fixing belt) having the elastic layer contain carbon fiber to enhance thermal conduction, and Japanese Patent Application Laid-Open No. 2000-39789 discloses a method of making the elastomer layer contain an anisotropic filler such as graphite to enhance the thermal conductivity in a roller thickness direction. In addition, Japanese Patent Application Laid-Open No. 2002-351243 discloses an invention of arranging the layer of woven fabric using pitch-based carbon fiber in the elastic layer of a pressure roller.
However, even if a filler such as alumina, zinc oxide, silicon carbide, carbon fiber and graphite as described in Japanese Patent Application Laid-Open No. H1l-116806, Japanese Patent Application Laid-Open No. H11-158377, Japanese Patent Application Laid-Open No. 2003-208052, Japanese Patent Application Laid-Open No. 2002-268423 and Japanese Patent Application Laid-Open No. 2000-39789 is added to an elastic layer for the purpose of increasing the thermal conductivity, a small amount of the addition can not provides a desired thermal conductivity, and a large amount of the addition causes a problem that a pressure roller becomes too hard to provide a sufficient nip for a toner-fixing process. In addition, when the hardness of a base rubber of forming an elastic layer is lowered so as to lower the hardness of the pressure roller while adding a large amount of the filler, the durability performance of the rubber becomes insufficient. Thus, it has been very difficult to balance high heat conduction with low hardness while keeping the durability performance of the pressure roller.
On the other hand, a pressure roller disclosed in Japanese Patent Application Laid-Open No. 2002-351243 has a very superior thermal conductivity. However, even the pressure roller has high hardness because of woven fabric contained in an elastic layer, and also very hardly balances high heat conduction with low hardness.